♻️ Simplify OpenQASM gate and entry-point naming

[burgholzer]

🤖 AI text below 🤖

Problem Statement

OpenQASM import normally names the generated entry function main. A custom gate may also be named main, so the importer renames the entry function to _mqt_entry and probes numbered suffixes against all custom gate names. This works, but spreads a source-name collision into the internal symbol policy and adds special-case code.

This follows up on the final review of #2338.

Proposed Solution

Define a simple internal naming contract that keeps the generated entry point distinct from source-level custom gate symbols. Preserve legal custom gate names, including main, across import and export while maintaining symbol-table uniqueness.

Remove the collision scan if the new invariant provides the same guarantee. Keep regression coverage for a custom gate named main and for collisions with reserved internal names.